Yes, i agree that the timeframe is unrealistic and that you progress in a too fast way. In Planescape Torment this is not such a problem due to that the guy you play isimmortal and founds lost knowledge as he ventures and learn things he already kne. He has been as high you can get in all classes..
Well the problem is to mke an AI that will play your advisaries in an inteligent and good way. For example, a couple of orcs fighting in a more organized way, being a bit tougher and has both fighters for close and ranged attacks and some magic/divine support that doesn´t just stand there but uses the environment to full advantage can be more than a match. This is easy to make true in tabletop but harder in CRPG´s..
BG2 gives alot, too much, mission XP´s for insatnace finding a teddybear for a couple of thousands worth of XP´s, (cute but not well good)
In IWD you also gains level quite fst but I got only to level 14 with my fighter in the end of the game.. So I think That IWD spans over a larger period an you progress more, from 1st to alot now.. But it feels like you have accomplished more than in BG2.
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Only the moon should hear my wail..